Rooney goes on the rampage to frighten the life out of Arsenal

Napoleon can hardly have retreated from Moscow with less dignity than Arsenal returned to London from Manchester. This drubbing exposed not only the team's inadequacy on the day in the face of a rampant United side who sensed miserable resistance almost from the kick-off, but also Arsène Wenger's tepid commitment to the FA Cup, whatever his ready-made complaints of depleted resources before and after.

His excuses were limp and unconvincing and did him and his team no great service when his normal candour would have been more appropriate.

This might have been United's easiest win of the season - in any competition - and Wayne Rooney was the man who inspired everyone from Nani, Anderson and Darren Fletcher marauding about up front, to a midfield whose command of territory and possession was almost total.

It was an embarrassingly one-sided victory. What the margin and the manner of the win also showed was that, for all Arsenal's quality, United are stronger throughout their squad going into the business end of the season. The five-point deficit in the League all of a sudden looks more like a hill than a mountain. As Sir Alex Ferguson pointed out, with some glee, it has to be said: 'Arsenal will know when they come here [on the weekend of 12-13 April], they are in for a game.'

Wenger, meanwhile, could offer no more than platitudes. 'Positive points are difficult to find today,' he said in that gnomic way of his that falls between irony and mischief. 'The only one I can see is there are not more injuries than before the game. We need to pick ourselves up quickly because we have a massive game on Wednesday night.' There you have it, of course. Arsenal play AC Milan at the Emirates in a Champions League fixture that so obviously means more to the club than the oldest Cup competition in football. Is that right or wrong?

Tell it to Rooney. The wild young man has matured into a player of manic commitment to the cause - in stark contrast to some of the truly awful football offered up by Arsenal's admittedly injury-hit squad. Wenger sought solace in the state of the pitch, which he described as 'a disgrace'. Nor was he convinced about the sending-off of Emmanuel Eboué after he had all but cut Patrice Evra in half in an aerial assault in the 49th minute that would have done the Red Baron proud.

But all that disguises the reality. United wanted this (Ferguson's 100th Cup tie) - even with Cristiano Ronaldo rugged up in the stands and Paul Scholes, Louis Saha, John O'Shea and Carlos Tevez on the bench. In Wenger's defence, his benched stars, Gaël Clichy, Mathieu Flamini and Emmanuel Adabeyor, were injured - although the latter two will face Milan.

It was a bigger humiliation even than Arsenal's 5-1 defeat to Tottenham in the Carling Cup last month. And United's comeback after drawing with Spurs and losing to Manchester City last weekend nailed suspicions of the infamous 'blip' so feared by Ferguson at this stage of the season. They are in the rudest of health, full of running and class.

'It was a marvellous performance,' Ferguson said. 'We passed really well and not many teams will beat Arsenal 4-0.'

Rooney started it and, had he been allowed to finish (United, too, have European commitments coming up, away to Lyon, who lost 1-0 at Le Mans yesterday), he would have ended the rout, too.

Apart from an early chance for Nicklas Bendtner, who had one-twoed neatly with Cesc Fábregas, there was not a moment when Arsenal were properly in the game.

Once United struck, it was horribly one-sided. Rooney, palely loitering, leapt at just the right time in the 16th minute to accept Anderson's simple nod forward, unchallenged, and popped it past the bemused Jens Lehmann.

Moments later, Fletcher went aerial, too, to post the simplest of goals from the simplest of crosses from Nani and Arsenal looked shattered. They would have been utterly devastated only a minute later had Rooney's header from Park Ji-Sung's lifted cross on the right not skimmed inches over.

Wenger was still remonstrating with a touchline official about some perceived wrong from several minutes earlier when United hit them with a third knockout blow, Nani leaving Lehmann for dead with a left-foot shot that snuck under him and into the far right corner.

By now Park and Nani were up alongside Rooney most of the time, so confident were United. They were playing with a freedom and energy that simply blew Arsenal away. When the Gunners returned a minute late after the half-time break, derision and mickey-taking filled the stadium. 'They've given up!' went the cry. They just about had.

Rio Ferdinand was booked as the commitment levels remained extraordinarily high and there was a hunger about United that allowed for no mercy. You had only to look at the expressions on their faces to see how Ferguson was loving it - and how Wenger was mentally curling up into a little ball. Park, perhaps not believing his luck, fluffed a chance near the hour, as his flailing boot failed to connect with Nani's glorious long cross.

Fletcher's commitment earned him a yellow card, but he rubbed in United's dominance with a straightforward header from Nani's cross with 16 minutes to go - and, at the other end, Adebayor was booked for diving.

When the end came, it was blessed relief for Arsenal and interruption to the fun for Manchester United. It was one of those wins that the victors can find tough to enjoy - because there was simply nothing coming back the other way.

Man of the match - Wayne Rooney

In a game so one-sided, invariably thewinners have players everywhere whoexcel. But the most obvious candidatetakes the honour: Rooney. He scored, hemight have scored again and again, and heran like the demented youth he is oftenperceived to be.